What controls the supply and flux of UDP-GlcA in plants is still debatable and it is very likely that different plant species adopt different mechanisms to control the supply of UDP — GlcA. In maize, mutants lacking the activity of one UDPGDH isoform have a reduction in the content of Ara, and Xyl in hemicellulose. This suggests that in maize, UDPGDH-A is a major supplier of the UDP-pentoses and that the myo-inositol oxidation pathway (not

“bifunctional ADH”) cannot compensate the flux of “sugar” to the formation of UDP-GlcA. In Arabidopsis mutants lacking MIOX1 and 2 isoform activities, no significant differences in monosaccharide amount or composition were observed in wall polysaccharides when compared to wildtype. Thus, it is likely that the major contributor for flux of NDP-sugars in plants is UDP-Glc. As mentioned above, it is possible that the myo-inositol pathway and the salvage pathway operate in a tissue-specific manner, for example, during pollen tube growth. During pollen tube germination and growth, large amounts of pectin are degraded. It is likely that the free sugars are recycled back by kinases and the activity of Sloppy, to readily form an available pool of NDP-sugars. This pool will provide NDP-sugars for growth of the pollen tube. Pollen tubes are one of the fastest growing cells known (1 cm h-1) and mutants lacking Sloppy have a pollen phenotype (415). A similar regulation scenario likely occurs during seed germination with seeds that store a large amount of phytic acid, as discussed above.